Many applications have very low data exchange rate, as low as once in 30 minutes. In such cases synchronization process according to existing technologies happens at a much faster rate, which is an expense on both battery power and spectral usage. As a result, in these cases there is a requirement that the terminal equipment matches its synchronization frequency with that of its data exchange. This will lead to the terminal equipment sleeping for longer durations and the durability of the battery is extended. Also, there is a need for the channel being used only as frequently as is required, for example, for data exchanges, as this increases the availability of spectrum for use by others. This helps in accommodating more number of networks in the same frequency band. These are highly desirable features for many wireless terminals and networks.
Two forms of synchronization are typical in a wireless communication system. Frequency synchronization refers to the adjusting of the receiver's transceiver chip's frequency generation system to match the sender's frequency generation system. This is required for correct reception of the signal at the RF level. Time synchronization refers to the establishment of a common reference for time amongst the entities of the network. Typically, time synchronization information is transmitted by a network coordinator (such as a base station) and other entities get synchronized with this information. Time synchronization requirements could be absolute or relative. In absolute time synchronization real world time is used for synchronization. The source for such time could be Global Positioning Systems (GPS). The network coordinator could have access to the GPS and distribute this time to the other entities of the network. In relative time synchronization it is sufficient to be able to time the events according to a network-wide reference, not an absolute time. The network coordinator could provide the reference. In either case, the network entities get synchronized with the time distributed by the network coordinator with a precision subject to the propagation delays. They continue to update this time locally and use it for timestamping their events. As the clocks of the entities keep drifting relatively, the synchronization task is repeated frequently. The time deviation of one clock relative to another clock, known as clock drift, occurs due to the physical nature of the quartz crystals used to implement the clocks.
In accordance with current network synchronization techniques, an element in the wireless network (NE) spends an appreciable amount of energy for time synchronization. The energy spent for synchronization is a big overhead for networks with low data exchange rate. An example of an application with low data exchange rate is human body monitoring, where specifics of various physical and biological parameters of the body can be transferred at intervals of 5 to 30 minutes. In such applications, in accordance with the current synchronization techniques, the NEs may be required to wake up more frequently only for time synchronization rather than for data exchange.
In accordance with IEEE 802.15.4 standard, an exemplary standard of a wireless communication network, viz., Wireless Sensor Network (WSN), time synchronization occurs through the beacon frames. All NEs time their actions with reference to the beacon frame. The beacons are transmitted periodically to ensure continued time synchronization. The NEs need to receive the beacon frames to keep in synchronization. The beacons also contain information for the NEs to time their receptions and transmissions. Every beacon frame, like all other frames, has a Preamble Data (PD) sequence of 32 bits present at the beginning. When the 2.4 GHz band is used the data rate is 250 Kbps. Thus the PD is transmitted for 128 microseconds (μs). NEs use the PD sequence to tune in to receive the rest of the frame, i.e., they have a window of 128 μs for tuning in. This determines the upper bound for the clock drift, at any NE, between two beacon transmissions by the NC. If the relative clock drift between the NC and an NE exceeds 128 μs between two beacon transmissions, then time synchronization is lost between the NC and the particular NE. For a clock with a drift of 40 parts per million (ppm), the NEs can drift by 128 μs in just 3.2 seconds. This will require the NC to send a beacon frame, to the NEs after every three seconds, keeping 0.2 seconds margin as a buffer. Consequently, the duty cycle (the beacon frame size transmitted per second) for time synchronization even for the shortest beacon frame size (544 μs) will be 181.333 μs per second. Compared to this, the data communication requirement taken at 100 bytes per 5 minutes would require the NE to be active only for 32 μs per second, at a data rate of 250 Kbps. Thus, the duty cycle for synchronization becomes higher than that of data exchange amongst the devices. In such a case, the NEs will have to become active more frequently for time synchronization than for data exchange. For a battery powered system where replacement of battery is impractical and, therefore, a significant operational overhead, frequent synchronization becomes a significant burden on the system